1. Field of the Invention
The present general inventive concept relates to a portable audio reproduction apparatus, and more particularly, to a method and apparatus to control an output power of a digital amplifier capable of automatically determining an impedance of an earphone or headphone connected to a portable audio device and enabling a power output to be optimized for the earphone or headphone.
2. Description of the Related Art
Recently, users of portable audio devices, such as MP3 players and portable multimedia players (PMPs), have demanded high quality sound. Also, the portable audio devices use digital audio amplifiers for low power consumption, smaller size, and high quality sound. Generally, A class, B class, AB class, and D class are used for digital audio amplifiers. Among these digital amplifiers, D class amplifier can reduce degradation of amplification efficiency occurring in the A, B, and AB class amplifiers. The D class amplifier converts an audio signal into a pulse width modulation (PWM) signal and switches the signal. In addition, a variety of earphones and headphones ranging from a 16Ω impedance earphone offered as a bundle item to a 120Ω impedance high-end headphone are being used in the portable audio devices.
Accordingly, in order to support the variety of earphones and/or headphones, an output capacity of a portable audio device should be large enough and a magnitude of an output signal should be set differently with respect to a type of the earphone and/or headphone being used in the portable audio device.
FIG. 1 is a block diagram illustrating a conventional digital amplification apparatus.
Referring to FIG. 1, a digital audio signal is output to an earphone/headphone 180 through a PWM unit 130, a power switching unit 150, and a low-pass filter 160.
An analog to digital converter (ADC) unit 120 converts a sound input to a microphone 122 into a digital signal in order to record the sound in a recorder. A microcomputer (micom) unit 110 generates different control signals and an ADC control signal to control respective blocks and units. A power supply unit 140 provides a predetermined direct current (DC) voltage (fixed VDC) to the power switching unit 150.
However, there occurs a large difference in magnitude (volume) of sound that a user hears according to a magnitude of an impedance of the earphone/headphone 180 connected to an output end of an amplifier. In general, an output power of an earphone or headphone at the magnitude of volume that a user hears most conveniently ranges from 5 mW to 7 mW.
For example, with reference to a 16Ω impedance headphone, a magnitude of a signal at the output end of the amplifier at which an output power of about 6 mW is obtained is 0.310 Vrms when the output signal is measured in a root square mean (RMS).
Accordingly, since most audio devices employ a 16Ω impedance earphone, an output level of the amplifier is set to 0.310 Vrms in order to output the output power of about 6 mW (approximately volume level 20).
However, since the output level of the amplifier is still set to 0.310 Vrms even when a 120Ω impedance high-end headphone is connected, an actual output power of the headphone may be limited to only about 0.8 mW. Accordingly, even though a user sets an identical volume level, if the impedance of the earphone/headphone 180 is increased, the sound output from the headphone/earphone 180 may become too soft to hear.
Similarly, if the volume level is set to deliver an output of about 6 mW with a 120Ω impedance high-end headphone, and the amplifier connection is then switched to a 16Ω impedance headphone, the user hears a very loud sound of about 44.7 mW and the user's ears may be hurt. FIGS. 2A and 2B are tables comparing headphone impedances, power consumptions in a headphone, and audible sound pressures with respect to magnitudes of signals output by the amplifier.
Therefore, according to the conventional digital amplification apparatus, unless the magnitude of the amplifier is appropriately adjusted with respect to the characteristics of the earphone or headphone impedance connected to the apparatus, the user may either not be able to hear the sound being amplified or may hear the sound at a decibel level that is too loud.